Check whether your Chromebook supports the Google Play Store
Open Settings
Turn on Google Play Store if it is available
Open the Google Play Store
Search for Standoff 2
Select the game from the search results
Click Install
Wait for the installation to finish
Open Standoff 2 from the app launcher or Play Store
Sign in if required
If the game is not available in the Play Store, update ChromeOS and try again
If your Chromebook does not support Android apps, Standoff 2 cannot be installed normally